Copper pillars are used in high density interconnect packages for electrically connecting a semiconducting micro-chip with an interposer, an IC substrate or a printed circuit board. Optionally, the copper pillars have a solder cap layer attached on the top portion of the copper pillars.
The copper pillars are replacing conventional solder ball bumps in such applications because they provide a fixed stand-off independent of the pitch (inter distance) between individual copper pillar structures.
FIG. 1 shows a method for manufacture of copper pillars according to the prior art. Copper pillars are usually manufactured on one side of a semiconductor substrate such as a micro-chip, referred herein as a die. The copper pillars (13) are formed by electroplating copper or a copper alloy into openings formed by a patterned resist material and onto an underbump metallization area (12) of a die (11) (FIG. 1). The underbump metallization area is a single layer comprising a metal less noble than copper, or a multilayer consisting of more than one individual layer of metals and/or metal alloys which are less noble than copper. The underbump metallization area has the main function of a barrier between copper and silicon and prevents undesired interdiffusion of said two elements.
Optionally, a solder material such as pure tin or a tin alloy such as tin-silver, tin-silver-copper or tin-silver-copper-nickel alloys is deposited onto the top portion of the copper pillars and thereby forming a solder cap layer (14). Such a method is for example disclosed in US 2005/0077624 A1.
The exposed side walls of the copper pillars are prone to undesired oxidation and/or other corrosion processes during further manufacturing steps of the die and the interconnect package.
A further disadvantage of such a copper pillar structure having a solder cap layer attached to the top portion is the good wettability of molten tin-based solder material on the side walls of the copper pillars. Accordingly, molten solder material can flow into the space between individual copper pillars during reflow operations and thereby undesired circuit shorts can be formed.
A method for manufacture of copper pillars having attached a solder material onto the top portion and having coated side walls is disclosed in US 2009/0127708 A1. The method disclosed therein comprises the steps of depositing a solder layer of tin or a tin alloy onto the top portion of the copper pillar and then depositing a thin protection layer (15) by immersion-type plating onto the entire outer surface of the copper pillar (FIG. 1).
The method disclosed in US 2009/0127708 A1 has a major disadvantage: undesired pin-holes are formed on the side walls of the copper pillars (13) when depositing the thin protection layer (15) onto the copper pillar (FIG. 2).
The undesired formation of pin-holes during immersion-type plating of a metal such as tin or silver is also observed for copper pillars (13) without a solder cap layer (14) of tin-based solder material on top of the copper pillar (13). Also in such a case the copper pillar (13) is formed on an underbump metallization area (12) which comprises at least one layer of a metal or metal alloy less noble than copper.